The present invention concerns an improved method for increasing the molecular weight of aliphatic polyester resins.
As is well known, aliphatic polyester resins are prepared using methods of melt polycondensation, under vacuum conditions, of aliphatic hydroxy-acids having two or more carbon atoms, or from the corresponding lactones or lactides, or from aliphatic dicarboxylic acids with diols having two or more carbon atoms.
When working under the conditions indicated above, it is difficult to obtain polymers having molecular weights high enough for the preparation of films having mechanical properties of practical importance.
It is known that the molecular weight can be taken to the required levels by reacting the melted resin with chain extenders such as the diisocyanates. In order to avoid the formation of gels, which normally occurs when a diisocyanate is melt reacted with an aliphatic polyester, the quantity of diisocyanate must be accurately controlled with respect to the terminal hydroxyl groups of the resin, and a resin that has an average numerical molecular weight greater than at least 5,000 must preferably be used.
If molecular weights that are too low are used, there is too much gel formation.
A greater disadvantage of using the diisocyanates at high temperatures is constituted by the branching and/or cross-linking reactions that can arise using these compounds.
The presence of gels makes the polymer unsuitable for applications such as film preparation.
It has now unexpectedly been discovered that it is possible to increase, even significantly, the molecular weight of aliphatic polyester resins without the formation of gels and/or uncontrolled cross-linking by conducting the reaction leading to the increase in molecular weight (a re-grading reaction) at temperatures at which the resin is in the solid state, and using bi-functional chain extenders that react with addition reactions with the terminal OH groups of the resin.
The re-grading reaction is conducted by purring the solid resin in granular form in contact with the chain extender and operating at ambient temperature or a temperature slightly less than the melting point of the resin for sufficient time to obtain the desired increase in molecular weight.
The chain extender is used in its liquid state, homogeneously dispersed over the resin so that it can diffuse therein.
Because the method is conducted in the solid phase at low temperature, relatively long time periods are needed to obtain significant increases in molecular weight. In order to reduce the times needed to obtain significant increases in the molecular weight, the chain extender is mixed in the melted polymer using relatively short contact times, generally less than five minutes; this is in order to avoid undesirable cross-linking reactions.
The increase in the intrinsic viscosity of the resins to which the re-grading process of the invention is applied is at least 0.1-0.15 dl/g. The intrinsic viscosity of the resin after the re-grading is greater than 0.7 dl/g.
The intrinsic viscosity of the starting polyester resins can vary within a wide range; for example, they can have low values such as 0.1-0.15 dl/g, or high values such as 0.8-1.0 dl/g, or higher such as, for example, 1.5 dl/g, the viscosity being measured in CHCl3.
The preferred chain extenders are the organic diisocyanates and polyisocyanates. They are used in sufficient quantities to react with the terminal OH groups of the resin.
Quantities of diisocyanate providing more NCO-groups than is equivalent to the number of OH groups of the resin have a negative effect on the increase in molecular weight, that is, in this case the diisocyanates do not react with both functional groups and, therefore, the reaction does not lead to an increase in molecular weight.
The quantity of diisocyanate or polyisocyanate is between 0.2 and 1 NCO equivalent per OH group of the resin.
The quantity of diisocyanate or polyisocyanate expressed in weight is generally between 0.01 and 3% of the resin, and preferably between 0.1 and 2%.
The organic diisocyanates and polyisocyanates are chosen from compounds such as the following:
a) compounds having the formula: OCN(CH2)nNCO, in which n is an integer between 2 and 20; tetramethylenediisocyanate, hexamethylenediisocyanate, dodecamethylenediisocyanate are representative compounds;
b) diisocyanates as in a), in which one or more of the hydrogen atoms of the CH2 groups are replaced by aliphatic radicals; 4-butylhexamethylenediisocyanate and 2,2,-4-trimethylhexamethylenediisocyanate are representative compounds;
c) diisocyanates such as in a) and b), in which one or more of the non-adjacent CH2 groups is replaced by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94NRxe2x80x94 groups, in which R is hydrogen or a hydrocarbon radical;
d) aromatic diisocyanates possibly having condensed rings, such as toluene-2,4-diisocyanate, p-phenylenediisocyanate;
e) diisocyanates having the formula OCN-A2-X-A2-NCO, in which A2 is a bivalent aryl radical, X is an xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, SO2xe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94COxe2x80x94 group, a bivalent hydrocarbon radical or a direct bond between the A2 groups; examples of these diisocyanates are 3,3xe2x80x2-, 4,4xe2x80x2- and 3,4xe2x80x2-diphenylmethanodiisocyanate, 2,2-diphenylpropanodiisocyanate, 4,4-diphenyldiisocyanate.
Dimers, trimers and tetramers of the diisocyanates can be used.
The aromatic diisocyanates can give rise to problems of colour in the polymer; on the other hand, the aliphatic diisocyanates are free from this disadvantage but are less reactive.
The preferred diisocyanates are hexamethylenediisocyanate, diphenylmethanodiisocyanate and isophoronediisocyanate.
Examples of other chain extenders that can be sued are the diepoxides and the dianhydrides of aromatic tetracarboxylic acids, such as pyromellitic dianhydride.
The aliphatic polyester resins to which the re-grading method of the invention is applied are resins containing terminal OH groups, and include aliphatic polyesters, aliphatic/aromatic co-polyesters, co-polyester ethers, polyester-ether-amides, urethane-polyesters and carbamide-polyesters, in which the polyester or the sequence or the repeating aliphatic units of the co-polymers are obtained from aliphatic dicarboxylic acids having 2-22 carbon atoms and from aliphatic, aromatic or cyclo-aliphatic diols having 2-22 atoms, or from hydroxy-acids having 2-22 carbon atoms, or from the corresponding lactones or lactides, or from mixtures of dicarboxylic acids with mixtures of diols or from mixtures thereof with mixtures of hydroxy-acids or lactones and lactides.
The preferred dicarboxylic acids are succinic acid and adipic acid; pimelic, suberic, azelaic, sebacic and brassilic acids; the preferred hydroxy-acids or lactones are 6-hydroxycaproate and 1xe2x80x2xcex5-caprolactone and the hydroxybutyric acids, hydroxyvalerianic acid, 9-hydroxy-nonoic acid, 10-hydroxy-decanoic acid and 13-hydroxy-tridecancarboxylic acid.
The diols utilised for the preparation of the resins from dicarboxylic acids are aliphatic, aromatic and cyclo-aliphatic diols having 2-22 carbon atoms.
Preferred diols are 1,2-ethandiol, 1,4-butandiol, 1,6-hexandiol, 1,7-heptandiol, 1,10-decandiol, 1,12-dodecandiol, 1,4-cyclohexandimethylol and 1,4-cyclohexandiol.
Preferred resins are poly-xcex5-caprolactone, polyethylene and polybutylene-succinate, polyhydroxybutyrate-hydroxyvalerate, polylactic acid, polyalkyleneadipate, polyalkyleneterephthalate, polyalkyleneadipate-xcex5-caprolactam, poly-xcex5-caprolactone/xcex5-caprolactam, polybutylene-adipate-co-terephthalate, poly(1,10-decandiyl-decandionate) and poly(1,10-decandiyl-nonandionate), poly-tetramethylenesebacate, poly-hexamethylenesebacate.
The preparation of the resins using melt polycondensation is conducted at temperatures of between 180 and 230xc2x0 C. in the presence of known catalysts such as, for example, tetraisopropyltitanium and titanium acetylacetonate. The molecular weight obtainable by means of polycondensation is generally not greater than 10,000.
It is also possible to conduct the polycondensation in the presence of polyfunctional compounds having three or more groups that are reactive with the terminal OH and COOH groups, such as trimethylolpropane, pentaerythritol and trimellitic anhydride.
The quantity of polyfunctional compound is generally between 0.1 to 5% in moles per 100 moles of dicarboxylic acid or hydroxy-acid.
The polymers that can be obtained using the process of the invention are substantially free of gels and have optimal mechanical characteristics.
They are utilisable other than in the preparation of films that can be single or multi-layered, mono- or bi-orientated, as well as in all applications in which aliphatic polyester resins are required that have relatively high intrinsic viscosities (greater than 0.7 dl/g). Examples of such applications are semi-expanded and expanded materials, moulded articles for the agglomeration of pre-expanded particles, fruit and vegetable containers, bottles for the grocery, cosmetic and pharmaceutical sectors, fishing nets, bags for organic refuse and grass cuttings and the like.
It has been found, and this constitutes a further aspect of the invention, that in the polymers that have been re-graded according to the method of the invention and that have an intrinsic viscosity greater than 0.7 dl/g, the ratio R between the intrinsic velocity and the melt flow index measured at 180xc2x0 under 5 kg of weight is greater than 0.1.